Auxiliary system for assisting a wireless terminal in determining its position from signals transmitted from a navigation satellite

ABSTRACT

A wireless terminal and auxiliary system are disclosed that enable the wireless terminal to determine its location based on signals transmitted from navigation satellites. In particular, the tasks of signal acquisition and signal processing required of a wireless terminal in the prior art are divided between the wireless terminal and the auxiliary system in accordance with the illustrative embodiment. The auxiliary system assists the wireless terminal by acquiring information about the satellites&#39; ephemerides, by partially processing it and by transmitting the partially processed information to the wireless terminal in a form that is useful to the wireless terminal. The wireless terminal then uses the partially processed information from the auxiliary system to assist the wireless terminal in acquiring the ranging signals from the navigation satellites quickly and when they are weak.

FIELD OF THE INVENTION

The present invention relates to satellite positioning systems ingeneral, and, more particularly, to a telecommunications-based auxiliarysystem that assists a wireless terminal in determining its position fromsignals transmitted from one or more navigation satellites.

BACKGROUND OF THE INVENTION

A satellite positioning system, such as the Global Positioning System("GPS"), comprises a constellation of satellites that transmit signalsthat can be used by a wireless terminal to determine, in well-knownfashion, the wireless terminal's position. Typically, the signalstransmitted by each satellite convey three types of information: (1)satellite trajectory data, (2) system timing, and (3) ranginginformation. When a wireless terminal can acquire the signals from threeor more satellites the wireless terminal can determine its positionthrough triangulation, as is well-known in the art. FIG. 1 depicts aschematic diagram of a satellite positioning system in the prior art.

Although a conventional wireless terminal can determine its positionwith some degree of accuracy, fluctuations in the ionosphere and theatmosphere and jitter in the transmitted signals themselves prevent aconventional wireless terminal from determining its position with a highdegree of accuracy. To mitigate the effects of these factors and thusimprove the degree of accuracy with which a wireless terminal canascertain its position, another satellite positioning system, typifiedby the Differential Global Positioning System ("DGPS"), was developed.FIG. 2 depicts a schematic diagram of a Differential Global PositioningSystem.

As is well-known in the prior art, DGPS comprises terrestrial referencereceiver 205, whose position is static and exactly known throughconventional survey techniques, in addition to satellite constellation203 and wireless terminal 201. The theory underlying DGPS is that whenwireless terminal 201 is in close proximity (e.g., within 50 miles) toterrestrial reference receiver 205, both wireless terminal 201 andterrestrial reference receiver 205 are expected to experience the sameionospheric and atmospheric fluctuations and signal jitter. Terrestrialreference receiver 205 uses the signals from satellite constellation 203to estimate its position, and, using its known exact position,calculates the error between its estimated position and its known exactposition. That error or "difference" is a vector that represents theinaccuracy of the estimated position from the ionospheric andatmospheric fluctuations and signal jitter. The difference vector isbroadcast by terrestrial reference receiver 205 to wireless terminal 201in real time. When wireless terminal 201 estimates is position throughconventional means, it uses the difference vector received fromterrestrial reference receiver 205 to subtract out the effects of theionospheric and atmospheric fluctuations and signal jitter.

FIG. 3 depicts a schematic diagram of a Tidget® satellite positioningsystem in the prior art. The wireless receiver in a Tidget system doesnot compute the position of the wireless terminal. Instead, the wirelessreceiver in a Tidget system acts like a wireless repeater in that itreceives the signals from the satellite constellation and then relaysthe unprocessed signals to a remote processing facility, which uses thesignals to determine the position of the Tidget wireless terminal. Anadvantage of a Tidget system is that is reduces the cost of the wirelessterminal by eliminating from the wireless terminal the expensivecircuitry that would otherwise be needed to compute the position of thewireless terminal. When it is more advantageous that a remote facilityknow the location of the wireless terminal than that the wirelessterminal know its own location, a Tidget system is advantageous in thatit relays, in effect, the position of the wireless terminal to theremote facility.

FIG. 4 depicts a schematic diagram of a Tendler® satellite positioningsystem in the prior art. A wireless terminal constructed in accordancewith this system comprises both the circuitry needed to determine itsposition from a satellite constellation and a wireless telephonetransmitter to transmit the determined position to another party via awireless telecommunications system.

Regardless of the advances made in satellite positioning systems,limitations still exist. Typically, the strength of the signals from thesatellite constellation is too attenuated in buildings and othershadowed environments for a wireless terminal to receive. Furthermore, awireless terminal can take several minutes to acquire the signals fromthe satellites it needs to determine its position.

SUMMARY OF THE INVENTION

Some embodiments of the present invention are capable of assisting awireless terminal in determining its position based on signalstransmitted from one or more navigation satellites. In particular, awireless terminal that is assisted by an embodiment of the presentinvention can be less expensive than wireless terminals in the priorart. Furthermore, a wireless terminal that is assisted by an embodimentof the present invention is able to receive and use weaker signals thanwireless terminals in the prior art; and still furthermore, a wirelessterminal that is assisted by an embodiment of the present invention iscapable of determining its location far more quickly that wirelessterminals in the prior art.

An auxiliary system in accordance with an embodiment of the presentinvention assists a wireless terminal by apportioning the tasks ofsignal acquisition and signal processing that are normally borne by thewireless terminal alone between the wireless terminal and the auxiliarysystem. In particular, the requirements normally imposed on a wirelessterminal in the prior art are off-loaded onto the auxiliary system thatcan provide useful information to the wireless terminal over a wirelesstelecommunications link.

It is possible to divide the signal acquisition and signal processingtasks between the wireless terminal and the auxiliary system becauseeach signal transmitted by each satellite in a satellite positioningsystem's constellation carries two distinct kinds of information thatare responsive to independent acquisition and independent processing.The two kinds of information are: (1) ranging information, and (2)information about the satellites' ephemerides.

The information about the satellites' ephemerides is the same for allreceivers, regardless of their position. In contrast, the ranginginformation, which indicates to the receiver its distance from eachsatellite, is location dependent and can be received only by thewireless terminal itself. Therefore, the auxiliary system can assist thewireless terminal by acquiring the information about the satellites'ephemerides, by partially processing it and by transmitting it to thewireless terminal in a form that is useful to the wireless terminal. Theauxiliary system cannot, however, acquire the ranging information forthe wireless terminal.

By having the auxiliary system acquire the information about thesatellites' ephemerides for the wireless terminal, the signalacquisition and signal processing demands of the wireless terminal arereduced. Furthermore, the wireless terminal can actually use thepartially processed information from the auxiliary system to assists thewireless terminal in acquiring the ranging signals quickly and when theyare weak.

When the wireless terminal is capable of providing the functionality ofa wireless telecommunications terminal (e.g., a cellular telephone, ahand-held data entry device, etc.), the circuitry for determining thewireless terminal's location, in accordance with some embodiments of thepresent invention, can be added to the wireless terminal for moderatelylittle cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a satellite positioning system, such asGPS, in the prior art.

FIG. 2 is a block diagram of a differential GPS system in the prior art.

FIG. 3 is a block diagram a Tidget-like system in the prior art.

FIG. 4 is a block diagram of a Tendler-like system in the prior art.

FIG. 5 is a block diagram of a satellite positioning system inaccordance with the illustrative embodiment of the present invention.

FIG. 6 is a block diagram of the auxiliary system shown in FIG. 5.

FIG. 7 is a block diagram of the wireless terminal shown in FIG. 5.

FIG. 8 is a block diagram of the field receiver shown in FIG. 7.

FIG. 9 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with one embodiment ofthe present invention.

FIG. 10 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with another embodimentof the present invention.

DETAILED DESCRIPTION

FIG. 5 depicts a drawing of a satellite positioning system in accordancewith an illustrative embodiment of the present invention. The satellitepositioning system depicted comprises wireless terminal 501, satelliteconstellation 503, auxiliary system 505 and timing source 507. Satelliteconstellation 503 is the Global Positioning System as is well-known inthe art and will not be further discussed. It will be clear to thoseskilled in the art how to make and use embodiments of the presentinvention that work with other satellite constellations.

The principal goal of the illustrative embodiment is to reduce thesignal acquisition and signal processing requirements of a conventionalwireless terminal so that a wireless terminal in accordance with theillustrative embodiment can determine its location more quickly and withweaker signals than wireless terminals in the prior art. In accordancewith the illustrative embodiment, the signal acquisition and signalprocessing requirements of wireless terminal 501 are reduced at theexpense of auxiliary system 505. In particular, the tasks of signalacquisition and signal processing required for a conventional wirelessterminal to determine its position are divided between wireless terminal501 and auxiliary system 505.

It will be clear to those skilled in the art how the signal processingtask can be divided between wireless terminal 501 and auxiliary system505, as partially processed signal information can be exchanged back andforth between the two through wireless telecommunications link 504 asneeded to achieve desirable division of the signal processing task.

It is possible to divide the signal processing task between wirelessterminal 501 and auxiliary system 505 because each signal transmitted byeach satellite in satellite constellation 503 carries two distinct kindsof information that are responsive to independent acquisition andindependent processing. The two kinds of information are: (1) ranginginformation, and (2) information about the satellites' ephemerides. Morespecifically, the GPS signal is modulated with digital information in amanner similar to how, for example, a cellular telephone's radio signalis modulated with voice data. Such information can be detected anddemodulated by any receiver adapted to do so. The informationreconstructed by the receiver is an exact replica of the informationmodulated onto the signal by the transmitter (except for unwanted errorsdue to noise, distortion, etc.) and is the same for all receivers,regardless of their position. This information shall be referred to as"information about the satellites' ephemerides."

In contrast, in a location system there is also important information inthe precise timing of the signal. The transmitter carefully adjusts thetiming of the transmitted signal according to some precise reference,such that the timing of the signal, as received by the receiver, carriesinformation about the distance between the transmitter and the receiver(and, therefore, about the receiver's position). Such information willbe different from receiver to receiver, and is only available at thereceiver itself This information shall be referred to as "ranginginformation."

For example, since each satellite in constellation 503 transmits asignal 502 that contains both kinds of information to both wirelessterminal 501 and auxiliary system 505, some or all of the informationabout the satellites' ephemerides is acquired by auxiliary system 505through antenna 553, even though the ranging information acquired byauxiliary system 505 is relevant to the position of auxiliary systemantenna 553 and not to the position of wireless terminal 501. However,auxiliary system 505 has approximate knowledge of the position ofwireless terminal 501 (for example, through knowledge of the cell andsector where a mobile is located); therefore, auxiliary system 505combines this knowledge with the acquired ranging information and withthe satellites' ephemerides information to compute an estimate of theranging information at the position of wireless terminal 501. Thisestimate, together with the satellites' emphemerides information, istransmitted, via wireless telecommunications antenna 551, to wirelessterminal 501 to assist wireless terminal 501 in acquiring and processingranging information.

Once the ranging information has been acquired by wireless terminal 501,wireless terminal 501 can use the satellite ephemeris information andranging information to determine its location, or wireless terminal 501can transmit the ranging information back to auxiliary system 505 sothat auxiliary system 505 can determine the location of wirelessterminal 501.

Because wireless terminal 501 is freed from the task of acquiring someor all of the information about the satellites' ephemerides and isadvantageously provided with an estimate of the ranging information, itcan be fabricated from less expensive technology that need only performthe easier task of acquiring and processing the ranging information witha priori knowledge of an estimated form of that information.Furthermore, because the satellite ephemerides information is modulatedonto the same carrier as the ranging information, the provision of thesatellites' ephemerides information to wireless terminal 501 enableswireless terminal 501 to remove the satellites' ephemerides informationfrom the satellite signal received through antenna 512 and, thereby,acquire the ranging information even under faded conditions of lowsignal-to-noise ratio that are inadequate for the operation of awireless terminal in prior art.

Auxiliary system 505 can be a terrestrial facility, an airborne facilityor an artificial satellite in orbit around the earth. Unlike aDifferential Global Positioning System's terrestrial reference receiver,however, the position of auxiliary system 505 need not remain static norneed its exact location be known.

FIG. 6 depicts a block diagram of the salient components of auxiliarysystem 505, which comprises: timing signal receiver 603, timing signalantenna 552, coarse location estimator 601, telecommunications systemmanager 617, GPS receiver 605, GPS receiver antenna 553, timing signalcalibrator 607, PRN synchronization estimator 609, demodulator 61 1,satellite visibility estimator 613, satellite Doppler estimator 615,telecommunications transmitter 619 and telecommunications antenna 551.

In general, auxiliary system 505 uses its GPS receiver to obtain fromeach satellite above the horizon both ranging information andinformation about the satellite's ephemeris, in wellknown fashion usingthe C/A or Coarse Acquisition code. It will be clear to those skilled inthe art how to make and use embodiments of the present invention thatuse the P(Y) or P code. In the process of obtaining the ranging andsatellite ephemeris information, auxiliary system 505 learns, amongother things: (1) the PRN synchronization from each satellite (i.e., theexact timing of the PRN code transmitted by each satellite), (2) theDoppler shift associated with each satellite, (3) which satellites areabove the horizon, and (4) the 50 bps modulated bit stream from eachsatellite. Auxiliary system 505 then transmits to wireless terminal 501,via a wireless telecommunications channel, for each satellite above thehorizon: (1) an estimate of the PRN synchronization, (2) an estimate ofthe Doppler shift, and (3) the 50 bps modulated bit stream.Collectively, this information will be called "Navigation Message Data."

When auxiliary system 505 is part of a wireless telecommunicationssystem that partitions a geographic area into a number of tessellatedareas called "cells," auxiliary system 505 knows which cell wirelessterminal 501 is in and, therefore, its rough location to within a fewmiles. When auxiliary system 505 has a rough idea (e.g., within a fewmiles) of the position of wireless terminal 501, auxiliary system 505can accurately estimate the PRN synchronization and Doppler shift asseen by wireless terminal 501.

Because the PRN synchronization estimate, the Doppler shift estimate andthe 50 bps modulated bitstream are perishable and only useful whenwireless terminal 501 and auxiliary system 505 are synchronized within afew GPS C/A code chips, both wireless terminal 501 and auxiliary system505 are advantageously synchronized to within 1 μs. To accomplish this,both wireless terminal 501 and auxiliary system 505 can receive a timingsynchronization signal from independent timing source 507, in well-knownfashion. Alternatively, auxiliary system 505 can contain a timing sourceand can transmit a synchronization signal to wireless terminal 501 overthe telecommunications channel.

For example, when auxiliary system 505 is part of a CDMA wirelesstelecommunications system and wireless terminal 501 is CDMA compliant,both auxiliary system 505 and wireless terminal 501 will be synchronizedto within 1 μs and timing source 507 is not needed. It will be clear tothose skilled in the art how to provide synchronization for wirelessterminal 501 and auxiliary system 505.

Returning to FIG. 6, when auxiliary system 505 is part of an IS-95 CDMAtelecommunications system, telecommunications system manager 617 informscoarse location estimator 601 of the cell in which wireless terminal 501is located. Furthermore, telecommunications system manager 617 caninstigate the process of locating wireless terminal 501 when, forexample, wireless terminal 501 is carried by a lost child. As anotherexample, a "911" emergency-services call from wireless terminal 501 canprovoke telecommunications system manager 617 to locate wirelessterminal 501 and direct emergency service personnel to the location ofwireless terminal 501. Another position-based service could enable aperson whose car had broken down to enter a code, such as *TOW, intowireless terminal 501. Wireless terminal 501 would then relay *TOW totelecommunications system manager 617, which would then ascertain theposition of wireless terminal 501 and establish a call between wirelessterminal 501 and the towing service that was closest to wirelessterminal 501. The disclosure of pending U.S. patent application Ser. No.08/784108, filed Jan. 15, 1997, entitled "Wireless Location Messaging,"is incorporated by reference.

Coarse location estimator 601 uses the information fromtelecommunications system manager 617 to produce an estimate of thelatitude and longitude of the location of wireless terminal 501, whichestimate could simply be the location of the center of the cell orsector containing wireless terminal 501.

Timing signal receiver 603 receives the same timing signal from timingsource 507 that is received by wireless terminal 501, when timing source507 is needed for synchronization. The locations of timing signalreceiver 603 and timing source 507 must be known with sufficientaccuracy to allow timing signal calibrator 607 to accurately determinethe timing signal delay between timing source 507 and timing signalreceiver 603, as well as the timing signal delay between timing source507 and wireless terminal 501. For example, the required timing accuracycould be 1 μsec, based on the coarse estimate of the location ofwireless terminal 501. Alternatively, timing signal receiver 603 couldreceive the timing signal from GPS constellation 503.

GPS receiver 605 receives a signal, via GPS receiver antenna 553, fromeach satellite in satellite constellation 503 above the horizon anddetermines each signal's exact time of arrival (i.e., its PRNsynchronization). Demodulator 611 demodulates each acquired signal torecover its 50 bps modulated bit stream. PRN synchronization estimator609 predicts the exact time of arrival of each C/A code signal from eachvisible satellite at wireless terminal 501 and uses these predictions toestimate the PRN sequence timing to be used by the field receiver inwireless terminal 501 for proper de-spreading of the respective C/A codesignals. It should be understood that although PRN synchronizationestimator 609 cannot determine the exact PRN sequence timing at wirelessterminal 501, a good estimate (e.g., one that is correct within 10 or 20chips) substantially reduces the number of trial PRN synchronizationsthat wireless terminal 501 would otherwise have to try.

Satellite visibility estimator 613 extracts the satellite ephemeris fromthe received modulation bit streams and estimates which satellites arevisible to wireless terminal 501 at its location. Similarly, satelliteDoppler estimator 615 extracts satellite ephemeris information from thereceived modulation bit streams and estimates which satellites arevisible to wireless terminal 501 at its location. Telecommunicationstransmitter 619 takes the satellite visibility estimate, the PRNsynchronization estimate for each satellite, the Doppler shift estimatefor each satellite and the 50 bps modulated bitstream for each satelliteand transmits to wireless terminal 501 over a telecommunications channelfor each satellite above the horizon: (1) an estimate of the PRNsynchronization, (2) an estimate of the Doppler shift, and (3) the 50bps modulated bit stream. It will be clear to those skilled in the arthow to make and use auxiliary system 505.

FIG. 7 depicts a block diagram of the major components of wirelessterminal 501, which comprises: terminal controller 710, user interface720, telecommunications transmitter 741, telecommunications receiver751, field receiver 753, timing receiver 755, duplexor 733 and antenna731, interconnected as shown.

Advantageously, but not necessarily, wireless terminal 501 is capable ofperforming all of the functionality associated with a typical wirelessterminal particular a user telephone,). In particular a user of wirelessterminal is advantageously capable of having a two-way voiceconversation through telecommunications transmitter 741,telecommunications receiver 751 and auxiliary system 505.

Because the Navigation Message Data is transmitted to wireless terminal501 from auxiliary system 505, the Navigation Message Data is receivedby wireless terminal 501 via telecommunications receiver 751.Telecommunications receiver 751 passes the Navigation Message Data toterminal controller 710, which, in turn, passes the Navigation MessageData to field receiver 753.

As discussed above, wireless terminal 501 also advantageously receivessystem timing for synchronization purposes. When the timing signal istransmitted from timing source 507, the timing signal is received bywireless terminal 501 via timing receiver 755. Timing receiver 755passes the timing signal to terminal controller 710 which, in turn,passes the timing signal to field receiver 753. Alternatively, when thetiming signal is transmitted from auxiliary system 505, (as is the casewhen wireless terminal 501 and auxiliary system 505 are part of a CDMAtelecommunications system) the timing signal is received bytelecommunications receiver 751 Telecommunications receiver 751 thenpasses the timing signal to terminal controller 710 which, in turn,passes the timing signal to field receiver 753.

In either case, field receiver 753 receives the timing information thatit needs without needing to derive it from satellite constellation 503.Furthermore, field receiver 753 also receives for each satellite abovethe horizon: (1) an estimate of the PRN synchronization, (2) an estimateof the Doppler shift, and (3) the 50 bps modulated bit stream, againwithout having received any of this information directly from satelliteconstellation 503.

Wireless terminal 501 also advantageously receives the direct sequencespread spectrum C/A code signals from satellite constellation 503 viafield receiver 753.

FIG. 8 depicts a block diagram of the major components of field receiver753 that process the C/A code signal from one satellite in satelliteconstellation 503. For pedagogical reasons, the functions of fieldreceiver 753 are depicted in FIG. 8 as separate functional blocks thatoperate on one C/A code signal. It will be clear to those skilled in theart that in many embodiments of the present invention field receiver 753will be an appropriately programmed general-purpose microprocessor ordigital signal processor that simultaneously operates on C/A codesignals from multiple satellites. It will also be clear to those skilledin the art that many of the functional blocks in FIG. 8 can besubstituted for by transform techniques.

In FIG. 8, SPS controller 821 advantageously receives the NavigationMessage Data and timing synchronization information from lead 761 andoutputs: (1) the PRN synchronization estimate to PRN code generator 819,(2) the Doppler shift estimate to Doppler correction 809, and the 50 bpsmodulation bit stream to mixer 815 and location computer 823, allappropriately synchronized. RF front end 801 receives the C/A codesignal from a satellite, filters out everything other than the band ofinterest and mixes it down to IF in well known fashion. A/D converter803 takes the mixed-down signal and samples it at twice the chippingrate of 1.023 MChips/sec. or more. PRN code generator 819 beginsgenerating the PRN code sequence at 1.023 MChips/sec., which PRN codesequence has a period of 1023 chips, as is well-known in the art. PRNcode generator 819 can also use the Doppler shift estimate to correctthe PRN code sequence chip rate for Doppler shift, but, because theDoppler shift on the PRN code sequence is usually very small, this neednot always be done. It will be clear to those skilled in the art whenPRN code generator 819 can neglect correcting for Doppler shift and whenit can not.

It will be understood by those skilled in the art how the signalprocessing functions performed by the blocks that follow A/D converter803 in FIG. 8 can also be performed in alternative embodiments usinganalog techniques. In such embodiments, field receiver 753 will bedescribed by a block diagram similar to the one of FIG. 8 except thatAID converter 803 will appear at a different point in the functionalsequence of blocks.

It should be understood that no guarantee is needed that the PRNsynchronization estimate be correct or that the first PRN code sequencefrom PRN code generator 819 be synchronized exactly. If it turns outthat the PRN code sequence from PRN code generator 819 is notsynchronized (as is determined by spectral analyzer 817), the PRN codegenerator 819 will use the PRN synchronization estimate as an educatedguess at finding the true synchronization through a progressive searchof synchronization positions near the estimate, in well-known fashion.

Mixer 805 multiplies the PRN code sequence and the digitized C/A codesignal and outputs the despread C/A code to lowpass filter 807. Lowpassfilter 807 advantageously reduces the bandwidth of the signal so that itcan be sampled at a lower rate. This allows Doppler correction block 809to ignore all but one out of every several samples it receives fromlowpass filter 807, so that the resulting number of samples per secondis at least the Nyquist rate needed for accurate representation of theoutput of lowpass filter 807, or twice the bandwidth occupied by theoutput of lowpass filter 807. Advantageously, the bandwidth is equal tothe largest Doppler shift observable in the signal (caused by therelative motion of the satellite with respect to wireless terminal 501)increased by the bandwidth occupied by the 50-bps signal itself. Forexample, the bandwidth occupied by the output of Lowpass filter 807 canbe 8 kHz, corresponding to a Nyquist rate of 16 kilosamples/s).

The Doppler shift caused by the relative motion of the satellite withrespect to wireless terminal 501 is comprised of two components: aDoppler shift caused by the relative motion of the satellite withrespect to ground (for which an estimate is included in the navigationmessage data) and a Doppler shift caused by the relative motion, if any,of wireless terminal 501 with respect to ground. Doppler correction 809takes the signal from lowpass filter 807 and corrects for the estimatedDoppler shift due to the relative motion of the satellite with respectto ground. This can be accomplished, in well-known fashion, through, forexample, frequency conversion techniques where the frequency of a localoscillator is adjusted to achieve the desired correction.

The output of Doppler correction 809 is fed into lowpass filter 811which advantageously further reduces the bandwidth of the signal so thatit can be sampled at a yet lower rate. Again, FIFO 813 can ignore allbut one out of every several samples it receives from lowpass filter811. The samples that are not ignored must occur at a rate that is atleast a Nyquist rate equal to twice the bandwidth occupied by the outputof lowpass filter 811. Advantageously, the bandwidth is equal to thelargest Doppler shift caused by the relative motion of wireless terminal501 with respect to ground increased by the bandwidth occupied by the50-bps signal itself. For example, the bandwidth occupied by the outputof lowpass filter 811 can be 500 Hz, corresponding to a Nyquist rate of1 kilosamples/s).

The output of lowpass filter 811 is fed into FIFO memory 813, whichdelays the signal for only so long as it takes auxiliary system 505 torecover the 50 bps modulated bit stream and forward it to SPS controller821. Typically, FIFO memory 813 need only delay the signal for, most, afew seconds. The output of FIFO memory 813 is fed into mixer 815 to bemixed with the carefully synchronized 50 bps modulated bit stream. Themixing operation will further despread the signal by removing the 50-bpsmodulation. As a result, the output of mixer 815 will be the unmodulatedsignal carrier, if a signal is present (i.e., if the PRN synchronizationis correct).

The output of mixer 813 is fed into spectral analyzer 817, whichperforms, for example, a discrete fourier transform in well-knownfashion. When the output of mixer 813 is a pure sinusoid (which isindicated by a spectral spike out of spectral analyzer 817), it meansthat PRN code generator 819 is perfectly in sync with the C/A codesignal from the satellite. When the output of mixer 813 is other than apure sinusoid (which is indicated by something other than a spectralspike out of spectral analyzer 817), it means that PRN code generator819 is not in sync with the C/A code signal and must try anothersynchronization. It will be clear to those skilled in the art how toperform the spectral analysis through techniques different than thosedescribed here, however, that yield the same result, which is detectingthe presence or absence of a narrowband component in the output of mixer815.

Importantly, when PRN code generator is in sync with the C/A code signalfrom the satellite, it means that location computer 823 can compute theranging information (i.e., how long did it take the signal to travelfrom the satellite to wireless terminal 501). And because locationcomputer 823 knows: (1) the PRN code synchronization from PRN codegenerator 819, (2) the modulated bit stream from SPS controller 821 and(3) when the PRN code is synchronized from spectral analyzer 817,location computer 823 can compute the location of wireless terminal 501,in well-known fashion.

The location of wireless terminal 501 can then be output from locationcomputer 823 to terminal controller 710 and to telecommunicationstransmitter 741 for transmission back to auxiliary system 505 over atelecommunications channel. Auxiliary system 505 can then use thelocation of wireless terminal 501 in any number of location-basedservices.

FIG. 9 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with one embodiment ofthe present invention.

FIG. 10 is a flowchart of the operation of the auxiliary system andwireless terminal shown in FIG. 5 in accordance with another embodimentof the present invention.

What is claimed is:
 1. An auxiliary system comprising:a satellitepositioning system receiver for receiving a signal from a satellite; aPRN code synchronization estimator for generating a PRN codesynchronization estimate based on said signal; and a telecommunicationstransmitter for transmitting said PRN code synchronization estimate to awireless terminal over a wireless telecommunications link.
 2. Theauxiliary system of claim 1 further comprising a Doppler shift estimatorfor generating a Doppler shift estimate, and wherein saidtelecommunications transmitter al its said Doppler shift estimate tosaid wireless terminal.
 3. The auxiliary system of claim 1 furthercomprising a demodulator for generating a modulation bit sequence, andwherein said telecommunications transmitter also transmits saidmodulation bit sequence to said wireless terminal.
 4. The auxiliarysystem of claim 1 further comprising a timing signal receiver forgenerating a system timing signal, wherein said telecommunicationstransmitter also transmits said system timing signal to said wirelessterminal.
 5. The auxiliary system of claim 1 further comprising a coarselocation estimator for generating a coarse location estimate of saidwireless terminal and wherein said PRN code synchronization estimate isbased on said signal and said coarse location estimate.
 6. The auxiliarysystem of claim 1 further comprising:a receiver for receiving apartially processed ranging signal from said wireless terminal; andmeans for determining a location of said wireless terminal based on saidpartially processed ranging signal.
 7. The auxiliary system of claim 1further comprising a receiver for receiving a location of said wirelessterminal from said wireless terminal.
 8. The auxiliary system of claim 7further comprising means for using said location of said wirelessterminal in a location-based service.
 9. An auxiliary systemcomprising:a satellite positioning system receiver for receiving asignal from a satellite; a demodulator for generating a modulation bitsequence based on said signal; and a telecommunications transmitter fortransmitting said modulation bit sequence to a wireless terminal over awireless telecommunications link.
 10. The auxiliary system of claim 9further comprising a Doppler shift estimator for generating a Dopplershift estimate, and wherein said telecommunications transmitter alsotransmits said Doppler shift estimate to said wireless terminal.
 11. Theauxiliary system of claim 9 further comprising a PRN codesynchronization estimator for generating a PRN code synchronizationestimate, and wherein said telecommunications transmitter also transmitssaid PRN code synchronization estimate to said wireless terminal. 12.The auxiliary system of claim 9 further comprising a timing signalreceiver for generating a system timing signal, wherein saidtelecommunications transmitter also transmits said system timing signalto said wireless terminal.
 13. The auxiliary system of claim 11 furthercomprising a coarse location estimator for generating a coarse locationestimate of said wireless terminal and wherein said PRN codesynchronization estimate is based on said signal and said coarselocation estimate.
 14. The auxiliary system of claim 9 furthercomprising:a receiver for receiving a partially processed ranging signalfrom said wireless terminal; and means for determining a location ofsaid wireless terminal based on said partially processed ranging signal.15. The auxiliary system of claim 9 further comprising a receiver forreceiving a location of said wireless terminal from said wirelessterminal.
 16. The auxiliary system of claim 15 further comprising meansfor using said location of said wireless terminal in a location-basedservice.
 17. An auxiliary system comprising:a timing signal receiver forgenerating a system timing signal; a satellite positioning systemreceiver for receiving a signal from a satellite; a demodulator forgenerating a modulation bit sequence; and a telecommunicationstransmitter for transmitting said system timing signal and saidmodulation bit sequence to a wireless terminal over a wirelesstelecommunications link.
 18. An auxiliary system comprising:a timingsignal receiver for generating a system timing signal; a satellitepositioning system receiver for receiving a signal from a satellite; aPRN code synchronization estimator for generating a PRN codesynchronization estimate based on said signal; and a telecommunicationstransmitter for transmitting said system timing signal and said PRN codesynchronization estimate to a wireless terminal over a wirelesstelecommunications link.
 19. The auxiliary system of claim 18 furthercomprising a coarse location estimator for generating a coarse locationestimate of said wireless terminal and wherein said PRN codesynchronization estimate is based on said signal and said coarselocation estimate.
 20. An auxiliary system comprising:a satellitepositioning system receiver for receiving a signal from a satellite; aDoppler shift estimator for generating a Doppler shift estimate based onsaid signal; a PRN code synchronization estimator for generating a PRNcode synchronization estimate a telecommunications transmitter fortransmitting said Doppler shift estimate and said PRN codesynchronization estimate to a wireless terminal over a wirelesstelecommunications link.
 21. An auxiliary system comprising:a satellitepositioning system receiver for receiving a signal from a satellite; aDoppler shift estimator for generating a Doppler shift estimate based onsaid signal; a demodulator for generating a modulation bit sequence atelecommunications transmitter for transmitting said Doppler shiftestimate and said modulation bit sequence to a wireless terminal over awireless telecommunications link.
 22. The auxiliary system of claim 20further comprising a coarse location estimator for generating a coarselocation estimate of said wireless terminal and wherein said PRN codesynchronization estimate is based on said signal and said coarselocation estimate.
 23. A method comprising:receiving a signal from asatellite with a satellite positioning system receiver; generating a PRNcode synchronization estimate based on said signal with a PRN codesynchronization estimator; and transmitting said PRN codesynchronization estimate to a wireless terminal over a wirelesstelecommunications link.
 24. The method of claim 23 furthercomprising:generating a Doppler shift estimate with a Doppler shiftestimator; and transmitting said Doppler shift estimate to said wirelessterminal.
 25. The method of claim 23 further comprising:generating amodulation bit sequence with a demodulator; and transmitting saidmodulation bit sequence to said wireless terminal.
 26. The method ofclaim 23 further comprising:generating a system timing signal with atiming signal receiver; and transmitting said system timing signal tosaid wireless terminal.
 27. The method of claim 23 further comprisinggenerating a coarse location estimate of said wireless terminal with acoarse location estimator, and wherein said PRN code synchronizationestimate is based on said signal and said coarse location estimate. 28.The method of claim 23 further comprising:receiving a partiallyprocessed ranging signal from said wireless terminal; and determining alocation of said wireless terminal based on said partially processedranging signal.
 29. The method of claim 23 further comprising receivinga location of said wireless terminal from said wireless terminal. 30.The method of claim 29 further comprising means for using said locationof said wireless terminal in a location-based service.
 31. A methodcomprising:receiving a signal from a satellite with a satellitepositioning system receiver; generating a modulation bit sequence basedon said signal with a demodulator; and transmitting said modulation bitsequence to a wireless terminal over a wireless telecommunications link.32. The method of claim 31 further comprising:generating a Doppler shiftestimate with a Doppler shift estimator; and transmitting said Dopplershift estimate to said wireless terminal.
 33. The method of claim 31further comprising:generating a PRN code synchronization estimate with aPRN code synchronization estimator; and transmitting said PRN codesynchronization estimate to said wireless terminal.
 34. The method ofclaim 31 further comprising:generating a system timing signal with atiming signal receiver; and transmitting said system timing signal tosaid wireless terminal.
 35. The method of claim 33 further comprisinggenerating a coarse location estimate of said wireless terminal with acoarse location estimator, and wherein said PRN code synchronizationestimate is based on said signal and said coarse location estimate. 36.The method of claim 31 further comprising:receiving a partiallyprocessed ranging signal from said wireless terminal; and determining alocation of said wireless terminal based on said partially processedranging signal.
 37. The method of claim 31 further comprising receivinga location of said wireless terminal from said wireless terminal. 38.The method of claim 37 further comprising using said location of saidwireless terminal in a location-based service.
 39. A methodcomprising:generating a system timing signal with a timing signalreceiver; receiving a signal from a satellite with a satellitepositioning system receiver; generating a modulation bit sequence basedon said signal with a demodulator; and transmitting said timing signalreceiver and said modulation bit sequence to said wireless terminal. 40.A method comprising:generating a system timing signal with a timingsignal receive; receiving a signal from a satellite with a satellitepositioning system receiver; generating a PRN code synchronizationestimate based on said signal with a PRN code synchronization estimator;and transmitting said timing signal receiver and said PRN codesynchronization estimate to said wireless terminal.
 41. The method ofclaim 40 further comprising generating a coarse location estimate ofsaid wireless terminal with a coarse location estimator, and whereinsaid PRN code synchronization estimate is based on said signal and saidcoarse location estimate.
 42. A method comprising:receiving a signalfrom a satellite with a satellite positioning system receiver;generating a Doppler shift estimate based on said signal with a Dopplershift estimator; generating a PRN code synchronization estimate with aPRN code synchronization estimator; and transmitting said Doppler shiftestimate and said PRN code synchronization estimate to said wirelessterminal.
 43. A method comprising:receiving a signal from a satellitewith a satellite positioning system receiver; generating a Doppler shiftestimate based on said signal with a Doppler shift estimator; generatinga modulation bit sequence with a demodulator; and transmitting saidDoppler shift estimate and said modulation bit sequence to said wirelessterminal.
 44. The method of claim 42 further comprising generating acoarse location estimate of said wireless terminal with a coarselocation estimator, and wherein said PRN code synchronization estimateis based on said signal and said coarse location estimate.
 45. Anauxiliary system comprising:a coarse location estimator for generating acoarse location estimate of a wireless terminal; a PRN codesynchronization estimator for generating a PRN code synchronizationestimate based on said coarse location estimate; and atelecommunications transmitter for transmitting said PRN codesynchronization estimate to a wireless terminal over a wirelesstelecommunications link.
 46. The auxiliary system of claim 45 furthercomprising a Doppler shift estimator for generating a Doppler shiftestimate based on said coarse location estimate, and wherein saidtelecommunications transmitter also transmits said Doppler shiftestimate to said wireless terminal.
 47. The auxiliary system of claim 45further comprising a demodulator for generating a modulation bitsequence, and wherein said telecommunications transmitter also transmitssaid modulation bit sequence to said wireless terminal.
 48. Theauxiliary system of claim 45 further comprising a timing signal receiverfor generating a system timing signal, wherein said telecommunicationstransmitter also transmits said system timing signal to said wirelessterminal.
 49. The auxiliary system of claim 45 further comprising asatellite positioning system receiver for receiving a signal from asatellite, and wherein said PRN code synchronization estimate is basedon said signal and said coarse location estimate.
 50. The auxiliarysystem of claim 45 further comprising:a receiver for receiving apartially processed ranging signal from said wireless terminal; andmeans for determining a location of said wireless terminal based on saidpartially processed ranging signal.
 51. The auxiliary system of claim 45further comprising a receiver for receiving a location of said wirelessterminal from said wireless terminal.
 52. The auxiliary system of claim51 further comprising means for using said location of said wirelessterminal in a location-based service.
 53. A method comprising:generatinga coarse location estimate of said wireless terminal with a coarselocation estimator; generating a PRN code synchronization estimate basedon said coarse location estimate with a PRN code synchronizationestimator; and transmitting said PRN code synchronization estimate to awireless terminal over a wireless telecommunications link.
 54. Themethod of claim 53 further comprising:generating a Doppler shiftestimate based on said coarse location estimate with a Doppler shiftestimator; and transmitting said Doppler shift estimate to a wirelessterminal over a wireless telecommunications link.
 55. The method ofclaim 53 further comprising:generating a modulation bit sequence with ademodulator; and transmitting said modulation bit sequence to saidwireless terminal.
 56. The method of claim 52 furthercomprising:generating a system timing signal with a timing signalreceiver; and transmitting said system timing signal to said wirelessterminal.
 57. The method of claim 53 further comprisingreceiving asignal from a satellite with a satellite positioning system receiver,and wherein said PRN code synchronization estimate is based on saidsignal and said coarse location estimate.
 58. The method of claim 53further comprising:receiving a partially processed ranging signal fromsaid wireless terminal; and determining a location of said wirelessterminal based on said partially processed ranging signal.
 59. Themethod of claim 53 further comprising receiving a location of saidwireless terminal from said wireless terminal.
 60. The method of claim59 further comprising means for using said location of said wirelessterminal in a location-based service.